Boat is winterized but didn't get the oil changed and probably
Won't happen, till spring. Should I leave 50 hr oil or would it be
Better to drain and leave it empty?

Depends on how much fuel, moisture and exhaust components got into it. Draining it and either leaving it empty or refilling it won't do much for you, since the new oil wouldn't mix with or replace what's in the engine, between the parts and on all of the surfaces. THAT'S where old oil can do some damage, but 50 hours really isn't a lot of time. If the engine idles a lot, it's more important but many people have left old oil in their engines for far too long and the damage won't be catastrophic unless certain conditions are met, which probably isn't your case.

The problem with old oil is the stuff that gets into it from running the engine, like JimN said. All of that will settle at some level in the oil and (worst case) will etch rust into the engine block or oil pan.

You are only going to leave it there for a short time (3-5 months), so I wouldn't worry about it. Just leave it until you can change the oil.

The best thing to do, would be to warm it up, change it, and run for a couple minutes. That way the contaminants don't rust things over the winter. That being said, 50hr modern oil, in a modern engine, is probably not going to hurt anything. It is just not the best you could do.....